.... because other people weren't speeding.
http://www.bbc.co.uk/news/10327666
Colin Montgomerie has history speeding (9 points on his licence already); gets caught doing 37 in a 30 zone; gets banned; then gets that ban overturned because the person using the speedgun also pointed it at people who weren’t speeding???? Seriously, logic bypass???
I do hope the next time there's a major murder case the police don't indiscriminately go through their DNA profiles and only target those who they really really really think might have done it...